1. Field of the Invention
The present invention relates to a mass flow rate-measuring method and a mass flow rate control apparatus using an orifice which operates as a sonic nozzle.
2. Description of the Related Art
Generally, it is especially difficult for an orifice to measure a minute flow rate of a fluid which is a gas. Specifically, there is a difference between the real mass flow rate (hereinafter referred to as xe2x80x9cQtrxe2x80x9d) and the theoretical mass flow rate (hereinafter referred to as xe2x80x9cQthxe2x80x9d) calculated from respective quantities of state.
Therefore, according to a conventional mass flow rate control apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-335117, the discharge coefficient Cd, which is required to use Qtr=Qthxc3x97Cd, is previously determined as a correspondence table Cd=f(Rth). Rth represents the theoretical Reynolds number.
Actually, the theoretical Reynolds number Rth and the theoretical mass flow rate Qth are calculated by detecting the pressure and the temperature upstream of the orifice. The discharge coefficient Cd, which corresponds to the theoretical Reynolds number Rth, is determined with reference to the correspondence table. The real mass flow rate is determined by the expression Qtr=Qthxc3x97Cd.
However, the inventor has found out the fact that the value of the discharge coefficient Cd differs depending on the type of the gas.
Therefore, for controlling the mass flow rate corresponding to a plurality of gas types by using the conventional mass flow rate control apparatus, it is necessary to previously store, in the memory, a correspondence table for the discharge coefficient Cd for each of the plurality of gas types. As a result, the memory capacity for storing the correspondence table is increased.
When the conventional mass flow rate control apparatus measures the temperature of he gas, a temperature-detecting element directly contacts the gas in a flow passage. However, the temperature-detecting element may be corroded and become defective depending on the type of the gas, making it impossible to use the mass flow rate control apparatus. The temperature-detecting element arranged in the flow passage may also disturb the flow of the gas.
It is an object of the present invention to provide a mass flow rate-measuring method and a mass flow rate control apparatus which do not unduly increase the memory capacity for storing a correspondence table, even when the number of gas types is increased.
Another object of the present invention is to provide a mass flow rate control apparatus which does not corrode a temperature-detecting element and which does not disturb the flow of gas in a flow passage.
According to the present invention, a plurality of respective relationships of discharge coefficient values classified by a physical property value of each of gases with respect to theoretical mass flow rates are previously determined. Therefore, an identical discharge coefficient relationship can be used for gas type in which the physical property values of the gases are similar to one another. Accordingly, even when the number of gas types is increased, it is unnecessary to drastically increase the memory capacity for storing a correspondence table necessary to determine a real mass flow rate.
According to the present invention, a storage means stores beforehand the determined respective relationships of a plurality of discharge coefficient values classified by a physical property value of each of gases with respect to theoretical mass flow rates. Therefore, an identical discharge coefficient relationship can be used for a gas type in which the physical property values of the gases are similar to one another. Accordingly, even when the number of gas types is increased, it is unnecessary to drastically increase the memory capacity for storing a correspondence table necessary to determine a real mass flow rate.
A gas temperature-detecting means detects a surface temperature of a metal structural member having therein a part of a flow passage. Accordingly, it is unnecessary to directly measure the temperature of a fluid. A temperature-detecting element of the temperature-detecting means does not become corroded, and the flow of the gas in the flow passage is not disturbed when the temperature is detected.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.